I've taken many stuck engines apart, including a 6 cyl tractor, that was setting outside, for 20 years with the vertical exhaust open.
The most highly penetrating solvent, is Xylene, next is Toluene. I mix equal parts Xylene, solvent and ATF. But, that would be expensive, to fill a 5 gal bucket. I would try adding a qt, of Xylene to the diesel.
If you take a brass hammer, to the valves, you'll be able to tell if they are stuck, by the sound. If they make a pop (valve head slapping the seat) you should be able to get them out w/o soaking the head. Just oil the valve stem, push it back and forth and unless it's mushroomed, it will come out w/o damage.